Flat panel displays are used in a wide variety of applications, such as televisions, notebook computers, projection systems, and cellular telephones.
FIGS. 1 and 2 shows a typical passive matrix flat panel display system 10 that includes a liquid crystal panel 12, in which a set of first transparent electrodes 22 are arranged in horizontal rows on the surface 18 of a first glass plate 14, and a set of second transparent electrodes 24 are arranged in vertical columns on the opposing surface 20 of a second glass plate 16, positioned parallel to and spaced apart from first glass plate 14. An electro-optical material 28, such as a liquid crystal, is sandwiched between plates 14 and 16; and a matrix comprising a large number of individually controllable picture elements 30, or "pixels," is defined wherever a first, or row, electrode 22 and a second, or column, electrode 24 overlap. Some displays, known as "dual-scan" displays, include two sets of column electrodes 24 separated by a non-conducting gap 40. Each set of column electrodes 24 overlaps only half of the total number of row electrodes 22, thereby forming two independently addressed display sections 42a and 42b, each comprising a separate, independent matrix of pixels 30.
A complete image is typically displayed during a time interval known as a "frame period," which lasts approximately one-sixtieth of a second. The optical state of each pixel 30, which is determined by the root mean square ("rms") of the potential difference between the row and column electrodes 22 and 24 over the frame period, is controlled by applying electrical addressing signals to row and column electrodes 22 and 24. The large number of pixels 30 allows the formation of arbitrary information patterns in the form of text or graphic images.
Addressing signals determined in accordance with any number of addressing techniques are applied to the electrodes by addressing signal voltage drivers, known as "LCD (liquid crystal display) drivers." Row electrodes 22 are typically "selected," i.e., have a nonzero, image-independent voltage applied, during one or more of the "addressing intervals" that comprise the frame period. Image-dependent column signals determined in accordance with the addressing technique are applied to the column electrodes 24 in each addressing interval. The application of the row addressing signals that cause selections of row electrodes 22 are coordinated with the application of the column signals to produce across each pixel during the addressing interval voltages that result in an rms voltage value over the frame period corresponding to the desired optical state of the pixel 30.
In some addressing techniques, row electrodes 22 are selected sequentially, a single row at a time, and each column signal during any addressing interval depends only upon the desired state of the pixel in that column corresponding to the single selected row. In more modern addressing techniques, such as Active Addressing.TM. techniques, multiple rows are simultaneously selected and each column signal is determined by the desired states of multiple pixels 30 in the column corresponding to the selected rows. Because each row is selected multiple times with the selections distributed over the frame period, image data corresponding to the multiple pixels must be available throughout the entire frame period to calculate the column signals. With such techniques, the rms value across individual pixels 30 during a frame period will be correct only if pixel values for the multiple pixels 30 in each column are not changed throughout the frame period. The entire frame of image data, therefore, must be stored and available for use in the calculations for a complete frame period. If the image data is changed during an frame period, the rms voltage will not be correct and image degradation will result.
A large quantity of data is associated with the image for each frame. A typical liquid crystal display may have 480 rows and 640 columns that intersect to form a matrix of 307,200 pixels. It is expected that matrix liquid crystal displays may soon comprise several million pixels. The state of each pixel, i.e., its color or shade of gray, is described by several bits of data, the exact number of bits depending upon the desired number of colors or gray levels. Because of the large number of pixels and multiple bits required to specify the optical state of each pixel, a large amount of image data is required to characterize the image of each frame.
Because it is not possible to receive an entire set of image frame data and calculate the column signals in the short period of time available between frames, it is generally necessary to have sufficient data storage associated with the display to store two complete frames of image data. One complete frame of image data is made available for calculating column signals, while image data for the subsequent frame is simultaneously being received and stored in another memory location.
To reduce the size, cost, and complexity of liquid crystal displays, it is desirable to reduce the amount of data storage required, but because modern addressing techniques require the same image data from multiple pixels in each column to be available during multiple addressing intervals, it has not been possible to reduce the data storage requirements for such displays below two complete frames.